JP5302840B2 - High-strength cold-rolled steel sheet with an excellent balance between elongation and stretch flangeability - Google Patents

Description

  The present invention relates to a high-strength steel plate excellent in workability, and more particularly, to a high-strength steel plate excellent in the balance between elongation (total elongation) and stretch flangeability.

  For example, steel sheets used for automobile frame parts and the like are required to have high strength for the purpose of collision safety and fuel efficiency reduction by reducing the weight of the car body, and excellent forming process for processing into complex frame parts However, depending on the parts to be processed, whether the emphasis is more on strength or the emphasis on workability is different.

  For this reason, provision of the high strength steel plate provided with the balance of elongation and stretch flangeability according to a strength class is desired.

  The inventors of the present invention have a tensile strength of 780 to 980 MPa grade steel sheets based on a dual-phase structure steel composed of ferrite and tempered martensite (see Patent Documents 1 and 2), but with a lower proportion of ferrite than these conventional steel sheets. In addition to reducing the hardness of the tempered martensite, and by refining the cementite particles precipitated in the martensite during tempering, the stretch flangeability is improved while ensuring the tensile strength. We have succeeded in developing a high-strength cold-rolled steel sheet with a high balance with elongation, and have already filed a patent application (see Patent Document 1).

  However, even a high strength steel sheet having a tensile strength of 1180 MPa class or higher, which is a higher strength class than the high strength cold rolled steel sheet described in Patent Document 1 (hereinafter referred to as “Prior Invention Steel Sheet 1”), is also stretched (all There is an urgent need to provide a high-strength steel sheet having an excellent balance between elongation (El) and stretch flangeability (hole expansion rate: λ).

  Therefore, the inventors of the present invention are based on a high-strength steel sheet having a two-phase structure composed of ferrite and tempered martensite (hereinafter sometimes simply referred to as “martensite”) as in the case of the above-described prior invention steel sheet 1. The ratio of ferrite is further reduced than that of the prior invention steel plate 1 (that is, the ratio of martensite is further increased), and the cementite in the martensite is maintained while maintaining the hardness of the martensite high by adjusting the tempering conditions. As a result of diligent study, it was considered that a high-strength steel sheet capable of satisfying the above-mentioned required level could be obtained by increasing the strength of martensite and improving the stretch flangeability. And has already filed a patent application (Japanese Patent Application No. 2009-203939, unpublished at the time of filing this application).

The invention according to the unpublished patent application is that the tensile strength [TS] is 1180 MPa or more and the balance between elongation and stretch flangeability [El × λ] can be ensured to be 1000% ·% or more in mass% (hereinafter referred to as chemical The same applies to the components.), C: 0.03 to 0.30%, Si: 0.1 to 3.0%, Mn: 0.1 to 5.0%, P: 0.1% or less, S: 0 Tempered martens having a composition of 1% or less, N: 0.01% or less, Al: 0.01-1.00%, the balance being iron and inevitable impurities, and having a hardness of more than 380 and not more than 450 Hv The site contains an area ratio of 70% or more (including 100%) and the balance has a structure made of ferrite, and the distribution state of cementite particles in the tempered martensite is equivalent to a circle equivalent diameter of 0.02 μm to 0.1 μm. Less than cementite particles Serial tempering martensite 1 [mu] m ² per 20 or more, the circle equivalent diameter 0.1μm or more cementite particles is a high strength cold rolled steel sheet is the tempered martensite 1 [mu] m ² 1.5 per below.

  However, it has excellent drawing workability compared to the prior invention steel plate 2 while securing a tensile strength substantially equivalent to that of the high strength cold rolled steel plate (hereinafter referred to as “prior invention steel plate 2”) according to the unpublished patent application. There is a further demand for a high-strength cold-rolled steel sheet (that emphasizes elongation) that has a tensile strength of 1130 MPa or more, TS × El of 16000 MPa ·% or more, and El × λ of 580% ·% or more. Yes.

JP 2004-256872 A JP 2004-232022 A JP 2009-144239 A

  Accordingly, an object of the present invention is to ensure a tensile strength substantially equal to that of the above-described prior invention steel plate 2 and excellent in drawing workability than the prior invention steel plate 2 (that is, more stretch is achieved in the balance between elongation and stretch flangeability). High-strength cold-rolled steel sheet, in other words, a high-strength cold-rolled steel sheet that is particularly excellent in drawing workability while ensuring a balance between elongation and stretch flangeability in a higher strength class than the steel sheet 1 of the prior invention. It is to provide.

The invention described in claim 1
% By mass (hereinafter the same for chemical components)
C: 0.1 to 0.3%
Si: 1.0-3.0%,
Mn: 0.1 to 5.0%,
P: 0.1% or less,
S: 0.1% or less,
N: 0.01% or less,
Al: 0.01 to 1.00%
And the remainder has a component composition consisting of iron and inevitable impurities,
A tempered martensite having a hardness of more than 380 and not more than 500 Hv is contained in an area ratio of 50% or more and less than 70%, and the balance has a structure made of ferrite with a hardness of 210 to 300 Hv,
The distribution state of cementite particles in the tempered martensite is
The cementite particles having an equivalent circle diameter of 0.02 μm or more and less than 0.1 μm are 20 or more per 1 μm ^{2 of the} tempered martensite,
A cementite particle having an equivalent circle diameter of 0.1 μm or more is 1.5 or less per 1 μm ^{2 of the} tempered martensite, and is a high-strength cold-rolled steel sheet having an excellent balance between elongation and stretch flangeability.

The invention described in claim 2
Ingredient composition further
Cr: 0.01-1.0% and / or Mo: 0.01-1.0%
The high-strength cold-rolled steel sheet having an excellent balance between elongation and stretch flangeability according to claim 1.

The invention according to claim 3
Ingredient composition further
Cu: 0.05-1.0% and / or Ni: 0.05-1.0%
The high-strength cold-rolled steel sheet having an excellent balance between elongation and stretch flangeability according to claim 1 or 2.

The invention according to claim 4
Furthermore,
Ca: 0.0005 to 0.01% and / or Mg: 0.0005 to 0.01%
The high-strength cold-rolled steel sheet having an excellent balance between elongation and stretch flangeability according to any one of claims 1 to 3.

  According to the present invention, in a two-phase structure composed of ferrite and tempered martensite, the hardness and area ratio of tempered martensite, the hardness of ferrite, and the distribution of cementite particles in the tempered martensite are appropriately controlled. By doing so, it became possible to ensure tensile strength while improving elongation in the balance between elongation and stretch flangeability. As a result, while securing a tensile strength substantially equivalent to that of the prior invention steel plate 2, the high strength steel plate superior in drawability than the prior invention steel plate 2, that is, in a higher strength class than the prior invention steel plate 1, While ensuring a balance between elongation and stretch flangeability, it has become possible to provide a high-strength cold-rolled steel sheet that is particularly excellent in drawing workability.

  The present inventors are based on a high-strength steel plate having a two-phase structure composed of ferrite and tempered martensite (hereinafter sometimes simply referred to as “martensite”) as in the case of the above-described prior invention steel plate 2. While the elongation is improved by increasing the proportion of ferrite (that is, decreasing the proportion of martensite) as compared with the steel plate 2 of the prior invention, the increase in the cold rolling rate in the cold rolling before annealing and the rapidness during annealing By preventing the softening of the work-hardened ferrite by heating and increasing the hardness of the ferrite, the tensile strength is ensured and the hardness of the martensite is kept high by adjusting the tempering conditions in the same manner as the steel sheet 2 of the prior invention. However, by reducing the cementite in the martensite and suppressing the decrease in stretch flangeability, it is possible to satisfy the above demand level. Considered degree steel plate can be obtained, extensive studies were carried out the results, which resulted in the completion of the present invention.

  Hereinafter, the structure characterizing the steel sheet of the present invention will be described first.

[Structure of the steel sheet of the present invention]
As described above, the steel sheet of the present invention has a two-phase structure composed of ferrite and tempered martensite, and is common to the steel sheet 2 of the prior invention in that the distribution state of cementite particles precipitated in the tempered martensite is the same. However, it is different in the following points.

  That is, the area ratio of the tempered martensite is 70% or more (including 100%) in the steel sheet 2 of the prior invention, whereas it is 50% or more and less than 70%, which is the less martensite side in the steel sheet of the present invention. Is limited to.

  Further, the hardness of the tempered martensite is controlled to be higher than 380 to 500 Hv, which is wider on the higher hardness side, whereas the hardness of the steel plate of the present invention is higher than 380 to 450 Hv in the prior invention steel plate 2.

  Further, the hardness of the ferrite is 130 to 181 Hv in the invention example and 208 Hv in the comparative example in the prior invention steel plate 2 (the estimation formula of HvF in paragraph [0027] of the specification of Japanese Patent Application No. 2009-203939 [this application It is calculated by substituting the component composition of [Table 1] into the formula (1) in the specification.], Whereas in the steel sheet of the present invention, it is controlled to 210 to 300 Hv which is a higher hardness side. Has been.

<Tempered martensite with a hardness of over 380 Hv and up to 500 Hv: 50% or more and less than 70% in area ratio>
In order to improve the elongation compared to the prior invention steel plate 2, the area ratio of tempered martensite is set to 50% or more and less than 70% smaller than the prior invention steel plate 2 (70% or more), and the remaining ferrite area ratio is set to the above prior invention. It is larger than the steel plate 2. In addition, by increasing the hardness of the ferrite (described later), even if the hardness of the tempered martensite becomes higher, the degree of decrease in stretch flangeability is reduced. The invention steel plate 2 (over 380 Hv to 450 Hv or less) is set to a width higher than 380 Hv and 500 Hv or less on the higher hardness side.

  A preferable range of the hardness of the tempered martensite is more than 420 Hv and 500 Hv or less. Moreover, the preferable range of the area ratio of tempered martensite is 55% or more and 65% or less.

<Hardness of 210 to 300 Hv ferrite: balance>
In order to compensate for the decrease in tensile strength caused by reducing the area ratio of tempered martensite as described above, the hardness of the remaining ferrite is from the above-described prior invention steel plate 2 (130 to 181 Hv in the invention example, 208 Hv at the maximum in the comparative example). High 210-300Hv.

<Cementite particles having an equivalent circle diameter of 0.02 μm or more and less than 0.1 μm: 20 or more per 1 μm ^{2 of} tempered martensite,
Cementite particles with an equivalent circle diameter of 0.1 μm or more: 1.5 particles or less per 1 μm ^{2 of} tempered martensite>
As with the prior invention steel plate 2, both the elongation and stretch flangeability can be improved by controlling the size and number of cementite particles precipitated in martensite during tempering. In other words, a large amount of moderately fine cementite particles are dispersed in martensite to increase the work hardening index by acting as a growth source of dislocations, contributing to improvement in elongation, and at the time of deformation of the stretch flange, Stretch flangeability can be improved by reducing the number of coarse cementite particles as starting points.

In order to effectively exhibit the above action, as in the case of the steel plate 2 of the prior invention, moderately fine cementite particles having an equivalent circle diameter of 0.02 μm or more and less than 0.1 μm are 20 or more per 1 μm ^{2 of} tempered martensite, preferably 25 or more, more preferably 30 or more, but coarse cementite particles having an equivalent circle diameter of 0.1 μm or more are 1.5 or less, preferably 1.3 or less, more preferably 1 μm ^{2 of} tempered martensite. Is limited to 1.0 or less.

  The reason why the lower limit of the equivalent circle diameter of the moderately fine cementite particles is set to 0.02 μm is that the finer cementite particles do not give sufficient strain to the martensite crystal structure, and the growth source of dislocations. It is because it is thought that it hardly contributes.

  Hereinafter, the hardness and area ratio of tempered martensite, the hardness of ferrite, and the method of measuring or estimating the size and number of cementite particles will be described. This is the same as the measurement method or estimation method described in 3.

  First, regarding the area ratio of martensite, each test steel sheet was mirror-polished, corroded with a 3% nital solution to reveal the metal structure, and then a scanning type with a magnification of 20000 times for approximately 4 μm × 3 μm region 5 fields of view. An electron microscope (SEM) image was observed, and the area ratio of martensite was calculated from the area ratio of each area, with the area not containing cementite being ferrite and the remaining area being martensite by image analysis.

  Next, regarding the hardness of ferrite, the following formula (1) (translated by Toshio Fujita et al .: “Design and Theory of Steel Materials” (Maruzen Co., Ltd.), issued on September 30, 1981, FIG. .1 was used to formulate the degree of influence (straight line) of the amount of each alloying element on the yield stress change of low C ferritic steel, and other elements such as Al and N were hardened by ferrite. The hardness HvF of the ferrite was estimated.

HvF = 102 + 209 [% P] +27 [% Si] +10 [% Mn] +4 [% Mo] −10 [% Cr] +12 [% Cu] Formula (1)
Here, [% X] is the content (mass%) of the component element X.

  Next, as for the hardness of martensite, the Vickers hardness (98.07N) Hv of the surface of each test steel sheet was measured according to the test method of JIS Z 2244, and the hardness of martensite using the following formula (2). Conversion to HvM was performed.

HvM = (100 × Hv−VF × HvF) / VM Expression (2)
Here, HvF: hardness of ferrite, VF: area ratio (%) of ferrite, and VM: area ratio (%) of martensite.

Regarding the size and the number of the cementite particles, each sample steel plate was mirror-polished and corroded with 3% nital to reveal the metal structure, and then the region inside the martensite was analyzed in a 100 μm ² region. A scanning electron microscope (SEM) image with a magnification of 10,000 times is observed for the field of view, and a white portion is marked as a cementite particle from the contrast of the image and marked, and the area of each marked cementite particle is circled by image analysis software. The equivalent diameter was calculated, and the number of cementite particles of a predetermined size existing per unit area was determined.

  Next, the component composition which comprises this invention steel plate is demonstrated. Hereinafter, all the units of chemical components are mass%.

[Component composition of the steel sheet of the present invention]
C: 0.1 to 0.3%
C is an important element that affects the area ratio of martensite and the amount of cementite precipitated in the martensite and affects the strength and stretch flangeability. If it is less than 0.1%, the strength cannot be ensured. On the other hand, if it exceeds 0.3%, the martensite hardness becomes too high and stretch flangeability cannot be ensured. The range of C content is preferably 0.12 to 0.25%, and more preferably 0.14 to 0.20%.

Si: 1.0-3.0%
Si contributes to ferrite strengthening as a solid solution strengthening element and has the effect of suppressing the coarsening of cementite particles during tempering, while preventing the formation of coarse cementite particles, and the number of moderately fine cementite particles. Is a useful element that contributes to both elongation and stretch flangeability. If it is less than 1.0%, the effect of strengthening ferrite is insufficient, and the increase rate of coarse cementite particles is excessive with respect to the increase rate of moderately fine cementite particles during tempering. On the other hand, if it exceeds 3.0%, the formation of austenite at the time of heating is inhibited, so the area ratio of martensite cannot be ensured and stretch flangeability cannot be ensured. The range of Si content becomes like this. Preferably it is 1.3 to 2.7%, More preferably, it is 1.5 to 2.5%.

Mn: 0.1 to 5.0%
Mn, like Si, has the effect of suppressing the cementite coarsening during tempering, and prevents the formation of coarse cementite particles, while increasing the number of moderately fine cementite particles, It is an element that contributes to both stretch flangeability and is useful for ensuring hardenability. If it is less than 0.1%, the increase rate of coarse cementite particles is excessive with respect to the increase rate of moderately fine cementite particles at the time of tempering. When it is too high, austenite remains at the time of quenching (at the time of cooling after annealing), and the stretch flangeability is deteriorated. The range of Mn content is preferably 0.30 to 2.5%, more preferably 0.50 to 2.0%.

P: 0.1% or less P is inevitably present as an impurity element and contributes to an increase in strength by solid solution strengthening, but segregates at the prior austenite grain boundaries and embrittles the grain boundaries to increase stretch flangeability. Since it deteriorates, it is made 0.1% or less. Preferably it is 0.05% or less, More preferably, it is 0.03% or less.

S: 0.1% or less S is also unavoidably present as an impurity element, forms MnS inclusions, and becomes a starting point of a crack when expanding a hole, thereby reducing stretch flangeability. . More preferably, it is 0.01% or less.

N: 0.01% or less N is also unavoidably present as an impurity element and lowers the elongation and stretch flangeability by strain aging, so the lower one is preferable, and the content is made 0.01% or less.

Al: 0.01 to 1.00%
Al combines with N to form AlN and reduces the solid solution N that contributes to the occurrence of strain aging, thereby preventing the stretch flangeability from deteriorating and contributing to the strength improvement by solid solution strengthening. If it is less than 0.01%, solute N remains in the steel, so strain aging occurs and elongation and stretch flangeability cannot be secured. On the other hand, if it exceeds 1.00%, austenite formation during heating is inhibited. The area ratio of martensite cannot be secured, and stretch flangeability cannot be secured.

  The steel of the present invention basically contains the above components, and the balance is substantially iron and impurities. In addition, the following allowable components can be added as long as the effects of the present invention are not impaired.

Cr: 0.01-1.0% and / or Mo: 0.01-1.0%
These elements are useful elements for increasing the precipitation strengthening amount while suppressing deterioration of stretch flangeability by precipitating as fine carbides instead of cementite. Addition of less than 0.01% of each element cannot effectively exert the above-described effect, while addition of more than 1.0% of each element results in excessive precipitation strengthening and high martensite hardness. It will become too long and flangeability will fall.

Cu: 0.05 to 1.0% and / or Ni: 0.05 to 1.0%
These elements are elements useful for improving the balance between elongation and stretch flangeability because it becomes easy to obtain moderately fine cementite by suppressing the growth of cementite. When less than 0.05% of each element is added, the above-described effects cannot be exhibited effectively. On the other hand, when more than 1.0% of each element is added, austenite remains at the time of quenching, and stretch flangeability is deteriorated. .

Ca: 0.0005 to 0.01% and / or Mg: 0.0005 to 0.01%
These elements are useful elements for improving stretch flangeability by miniaturizing inclusions and reducing the starting point of fracture. If less than 0.0005% of each element is added, the above effect cannot be exhibited effectively. On the other hand, if more than 0.01% of each element is added, inclusions are coarsened and stretch flangeability is lowered. To do.

  Next, the preferable manufacturing method for obtaining this invention steel plate is demonstrated below.

[Preferred production method of the steel sheet of the present invention]
In order to manufacture the cold-rolled steel sheet as described above, first, steel having the above composition is melted and hot rolled after being formed into a slab by ingot forming or continuous casting. As hot rolling conditions, the finishing temperature of finish rolling is set to Ar ₃ point or higher, and after appropriate cooling, winding is performed in a range of 450 to 700 ° C.

  After the hot rolling is finished, pickling is performed and then cold rolling is performed. In order to increase the hardness of the ferrite by work hardening, the cold rolling rate (also referred to as cold rolling rate) is the above-described prior invention steel plate 2 (30). %) Is preferably 50 to 80%.

  Then, after the cold rolling, annealing and further tempering are performed.

[Annealing conditions]
As annealing conditions, in order to prevent the softening of the work-hardened ferrite by the cold rolling of the previous stage and to secure the hardness of the ferrite, the annealing heating temperature is increased from (Ac1 + 20 ° C.) to Ac3, unlike the steel plate 2 of the prior invention. After rapid heating at a temperature rate of 1 to 10 ° C./s and holding for a short time of annealing holding time: 300 s or less, from the annealing heating temperature to a temperature directly below the Ms point, 50 ° C./s or more, similar to the preceding invention steel plate 2 It is better to quench at a cooling rate of.

<Annealing heating temperature: (Ac1 + 20 ° C.) to Ac3, heating rate 1 to 10 ° C./s, annealing holding time: 300 s or less>
Rapid heating during annealing and holding for a short time at the two-phase region heating temperature prevents transformation of the work-hardened ferrite during cold rolling while transforming a predetermined amount to austenite, followed by austenite during cooling This is because the area ratio of martensite produced by transformation from 50% or more and less than 70% is ensured.

  When the annealing heating temperature is less than (Ac1 + 20 ° C.), the amount of transformation to austenite is insufficient during annealing heating, so that the amount of martensite transformed from austenite during subsequent cooling is reduced, and an area ratio of 50% or more can be secured. On the other hand, if it exceeds Ac3, the austenite structure is coarsened and the bendability and toughness of the steel sheet are deteriorated, and the annealing equipment is deteriorated.

  On the other hand, if the rate of temperature rise is less than 1 ° C./s, the work-hardened ferrite is softened during cold rolling and the hardness of the ferrite cannot be secured. On the other hand, if it exceeds 10 ° C./s, the ferrite softening is prevented. The effect will be saturated.

  On the other hand, if the annealing holding time exceeds 300 s, the ferrite that has been work-hardened during cold rolling is softened, and the hardness of the ferrite cannot be ensured.

<Rapid cooling at a cooling rate of 50 ° C./s or higher to a temperature below Ms>
This is because a martensite structure is obtained by suppressing the formation of a ferrite or bainite structure from austenite during cooling.

  When the rapid cooling is finished at a temperature higher than the Ms point or when the cooling rate is less than 50 ° C./s, bainite is formed, and the strength of the steel sheet cannot be secured.

[Tempering conditions]
As tempering conditions, from the temperature after the annealing cooling to the tempering heating temperature: 500 to 600 ° C., heated at an average heating rate of 10 ° C./s or more, tempering holding time: kept within 30 s, then 10 ° C./s or more. What is necessary is just to cool at an average cooling rate.

  This is because, by tempering at a high temperature for a short time, both the reduction of the martensite hardness due to the reduction of the dislocation density in the martensite and the coarsening of the cementite particles in the martensite are prevented. In order to realize tempering for a short time at a high temperature, it is necessary to raise and lower the temperature at a certain rate.

  If the tempering heating temperature is less than 500 ° C, the tempering is insufficient and the strength is high, but the elongation and stretch flangeability are deteriorated. Elongation and stretch flangeability are reduced. In addition, when the average heating rate and the average cooling rate are less than 10 ° C./s, the cementite particles are coarsened. However, a speed exceeding 20 ° C./s is not necessary because the above effect is saturated.

Steels having the components shown in Table 1 below were melted to produce 120 mm thick ingots. This was hot rolled to a thickness of 25 mm, and then hot rolled again to a thickness of 3.2 mm. After pickling this, as shown in Table 2 below, the cold rolling rate was changed variously and cold rolled to obtain a test material, which was heat-treated under the annealing and tempering conditions shown in the same table.

  About each steel plate after heat treatment, the area ratio of martensite and its hardness, the hardness of ferrite, and the size of cementite particles by the measurement method or the estimation method described in the above-mentioned section of [Mode for Carrying Out the Invention] And measurement or estimation of the existence number.

  Moreover, about each said steel plate, tensile strength TS, elongation El, and stretch flangeability (lambda) were measured. The tensile strength TS and elongation El were measured in accordance with JIS Z 2241 by preparing a No. 5 test piece described in JIS Z 2201 with the long axis perpendicular to the rolling direction. Moreover, stretch flangeability (lambda) performed the hole expansion test according to the iron continuous standard JFST1001, and measured the hole expansion rate, and made this the stretch flangeability.

  The measurement results are shown in Tables 3 and 4.

  As shown in these tables, Steel No. 2 to 4, 9, and 16 to 25 all have a tensile strength TS of 1130 MPa or more, TS × El of 16000 MPa ·% or more, and El × λ of 580% ·% or more. ] A high-strength cold-rolled steel sheet satisfying the desired level described in the section above and having excellent drawing workability (that is, placing more emphasis on elongation) than the steel sheet 2 of the prior invention was obtained.

  On the other hand, steel No. which is a comparative example. 1, 5-8, 10-13, 15, 26-28 are inferior in any of the characteristics.

  Among these comparative examples, Steel No. 1, 5-8, 10, 11, 13 do not satisfy at least one of the requirements for defining the structure of the present invention because the manufacturing conditions are out of the recommended range, and the tensile strength [TS], tensile strength And / or the balance of elongation [TS × El] and at least one of the balance of elongation and stretch flangeability [El × λ] are inferior.

  That is, Steel No. No. 1 has a cold rolling rate that is too low, resulting in insufficient ferrite hardness and poor tensile strength.

  Steel No. In No. 5, since the annealing heating temperature is too low, the martensite area ratio is insufficient (that is, the ferrite area ratio becomes excessive), and the balance between tensile strength and elongation and stretch flangeability is inferior.

  On the other hand, Steel No. In No. 6, since the annealing heating temperature is too high, the ferrite hardness is lowered, and the balance between tensile strength and elongation and stretch flangeability is inferior.

  Steel No. 7 has a structure in which bainite is mixed because the annealing cooling rate is too low, the tensile strength is insufficient, and the balance between the tensile strength and the elongation is inferior.

  Steel No. In No. 8, since the tempering heating rate is too low, the cementite particles are coarsened, the stretch flangeability is lowered, and the balance between elongation and stretch flangeability is inferior.

  Steel No. In No. 10, since the tempering holding time is too long, the cementite particles are coarsened, the stretch flangeability is lowered, and the balance between elongation and stretch flangeability is inferior.

  Steel No. In No. 11, the tempering cooling rate is too low, so that cementite particles are coarsened and stretch flangeability is lowered, and the balance between stretch and stretch flangeability is poor.

  Steel No. In No. 13, since the tempering heating temperature is too high, the hardness of both martensite and ferrite is too low, the tensile strength and stretch flangeability are both lowered, and the balance between stretch and stretch flangeability is poor.

  Steel No. Nos. 26 to 28 do not satisfy at least one of the requirements for defining the structure of the present invention because the component composition of the steel is outside the specified range of the present invention, and the tensile strength [TS] and the elongation and elongation. At least one of the balance of flangeability [E1 × λ] is inferior.

  That is, Steel No. No. 26 has too low C content, resulting in insufficient martensite hardness and inferior tensile strength.

  Steel No. No. 27, when the Si content is too low, the ferrite hardness is insufficient and the tensile strength is lowered, and the cementite particles are coarsened and the stretch flangeability is lowered, and the balance between stretch and stretch flangeability is inferior. .

Steel No. In No. 28, the C content is too high, the martensite hardness becomes too high, the stretch flangeability is lowered, and the balance between elongation and stretch flangeability is inferior.

Claims (4)

% By mass (hereinafter the same for chemical components)
C: 0.1 to 0.3%
Si: 1.0-3.0%,
Mn: 0.1 to 5.0%,
P: 0.1% or less,
S: 0.1% or less,
N: 0.01% or less,
Al: 0.01 to 1.00%
And the remainder has a component composition consisting of iron and inevitable impurities,
A tempered martensite having a hardness of more than 380 and not more than 500 Hv is contained in an area ratio of 50% or more and less than 70%, and the balance has a structure made of ferrite with a hardness of 210 to 300 Hv,
The distribution state of cementite particles in the tempered martensite is
The cementite particles having an equivalent circle diameter of 0.02 μm or more and less than 0.1 μm are 20 or more per 1 μm ^{2 of the} tempered martensite,
A cementite particle having an equivalent circle diameter of 0.1 μm or more is 1.5 or less per 1 μm ^{2 of the} tempered martensite. A high-strength cold-rolled steel sheet having an excellent balance between elongation and stretch flangeability. Ingredient composition further
Cr: 0.01-1.0% and / or Mo: 0.01-1.0%
The high-strength cold-rolled steel sheet having an excellent balance between elongation and stretch flangeability according to claim 1. Ingredient composition further
Cu: 0.05-1.0% and / or Ni: 0.05-1.0%
The high-strength cold-rolled steel sheet having an excellent balance between elongation and stretch flangeability according to claim 1 or 2. Furthermore,
Ca: 0.0005 to 0.01% and / or Mg: 0.0005 to 0.01%
The high-strength cold-rolled steel sheet excellent in the balance of elongation and stretch flangeability according to any one of claims 1 to 3.